1. Field of the Invention
The invention generally relates to WLAN (wireless local area network) receivers, and in particular to functional units in such WLAN receivers.
2. Description of the Related Art
Flexibility is an important feature for modern data communication systems. This flexibility is, for example, offered by a wireless local area network implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems are offering the kind of mobility which allows the WLAN users to have access to real time information anywhere. This mobility supports productivity and service opportunities not possible with wired networks. Moreover, wireless data transmission simplifies the installation of a LAN system and provides a wide range of scalability.
To transmit data using radio waves in a WLAN the data being transmitted is superimposed onto a carrier wave. This process is called modulation. Today, most WLAN systems modulate carrier waves using spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. This technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two different spread spectrum modulation types are offered: frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).
FHSS systems hop from frequency to frequency in a pattern known to both the transmitter and receiver. DSSS systems create a redundant bit pattern called a chip or chipping code, for each transmitted bit. The transmitter and receiver both know the chipping code and are thus able to filter out signals that do not use the same bit pattern.
DSSS systems use bandwidth more efficiently than FHSS systems. Consequently, WLAN systems based on DSSS generally have higher throughput than their FHSS counterparts.
A standard for WLAN operations at data rates up to 2 Mbps in the 2.4-GHz ISM (Industrial, Scientific and Medical) band, is the IEEE 802.11 standard. To offer a higher bandwidth, the standard IEEE 802.11b was defined for data rates up to 11 Mbps in the 2.4-GHz ISM band and furthermore the IEEE 802.11a standard for data rates up to 54 Mbps in the 5-GHz Unlicensed National Information Infrastructure (UNII) band.
The IEEE 802.11 standard for wireless LANs using direct sequence spread spectrum technique employ a training preamble to train a receiver to a transmitter. Each transmitting data message comprises an initial training preamble followed by a data field. The preamble includes a synchronization field to ensure that the receiver can perform the necessary operations for synchronization. For the preamble length, two options have been defined, namely a long or a short preamble. All 802.11b compliant systems have to support a long preamble. The short preamble option is provided in the standard to increase the network throughput when transmitting special data such as voice and video. The synchronization field of a preamble consists of 128 bits for a long preamble and 56 bit for a short preamble.
Synchronization is one crucial aspect of the receiver. There are several methods to deal with the synchronization task. One approach was to use digital signal processing (DSP) providing high speed mathematical functions that can slice up in many small parts and analyze the spread spectrum signal to synchronize and decorrelate it. Another approach was to use applications specific integrated circuits (ASIC) as ASIC chips drive down the costs by using VLSI technology and creating generic building blocks that can be used in any type of application the designer wishes.
When operating a WLAN receiver, code synchronization is necessary because the code is a key to despreading the desired information. A good synchronization is achieved when the coded signal arrived in the receiver is accurately timed in both its code pattern position and sample selection.
For synchronization, the receiver detects the synchronization symbols and aligns the receiver's internal clock to the symbols in the synchronization field in order to establish a fixed reference time frame with which to interpret the fields in the transmission frame structure following the preamble. The preamble, including the synchronization field, is transmitted at the start of every message (data packet).
Another aspect of WLAN communication is, that in a mobile radio channel the signal level received at the antenna depends strongly on the location of the reception point. There can be large variations in the signal level over rather short distances. These signal variations can lead to situations where a receiver with a single antenna cannot receive a sufficiently strong signal to deliver acceptable reference. On the other hand, if more than one antenna is used, the chance that at least one antenna receives a sufficiently strong signal is increased. The approach of using several antennae that are spaced apart is called space or antenna diversity.
When several antennae are employed to pick up the radio signal there needs to be a mechanism to combine the signals that arrive at the antenna elements. A simple and cost effective approach is to select the antenna with the highest received signal power. Other approaches such as combining techniques may yield a higher performance but they need more than one RF and base band part. This is a requirement that considerably drives up system costs.
The acquisition problem is one of searching throughout a region of time and frequency (chip, carrier) in order to synchronize the spread spectrum signal with the locally generated sequence. Since the despreading process typically takes place before a carrier synchronization, and therefore the carrier is unknown at this point, most acquisition schemes utilize non-coherent detection.
FIG. 1 shows a block diagram of a prior art WLAN receiver 100. Via one or more antennae 110 the receiver receives a data stream from a WLAN transmitter and feeds the antenna output to a signal processing unit 120. In the signal processing unit the received data signals are preprocessed and handed over to the synchronization unit 130. After synchronizing the received data signals the synchronized data signals are handed over to the digital signal processing unit 140 for further digital signaling processing. The antenna selection is done by the antenna diversity controller or finite state machine. Its purpose is to measure at the beginning of said preamble which antenna delivers the strongest signal. This antenna will be the receive antenna for the frame. After selecting the antenna the preamble is detected by a preamble detection unit that scans the incoming data stream for a preamble while the receiver is in the receive mode. Its purpose is to detect a preamble and to determine whether a short or a long preamble is being received. It will also determine the boundaries between consecutive Barker symbols such that the following processing blocks can adjust their processing schedule accordingly. Finally, it will deliver an initial frequency error estimate that will be used in the frequency error correction module for an initial frequency error correction. Moreover, a synchronization unit performs a non-coherent detection to find the start of frame delimiter that divides preamble and header.
Due to this wide range of different tasks the synchronization circuits in existing WLAN receivers are very complex. As the digital signal processing functions need a plurality of functional units the circuits are highly involved. Therefore the costs of circuit development and manufacturing are high.